The events of Sep. 11, 2001 forced recognition of an urgent need for more effective and stringent screening of airport baggage. The need for security expanded from the inspection of carry-on bags for knives and guns to the complete inspection of checked bags for a range of hazards with particular emphasis upon concealed explosives. The demonstrated willingness of terrorists to die in the pursuit of their aims meant that 100% passenger-to-bag matching, which could be put in place rapidly, was not sufficient to counter an attempt to conceal explosives in checked baggage and bring down an airliner. Successful screening for the presence of explosives presents numerous technological challenges, many of which are not met in present systems. X-ray imaging is the most widespread technology currently employed for screening. Last year approximately 1100 x-ray explosives detection systems incorporating computerized tomography (CT) scanners were purchased by the Transportation Security Agency (TSA) in an accelerated procurement program directed toward a goal of 100% screening of checked bags by Dec. 31, 2002.
Existing x-ray baggage scanners, including CT systems, designed for the detection of explosive and illegal substances are unable to discriminate between harmless materials in certain ranges of density and threat materials like plastic explosive. Thus, depending upon the level of the sensitivity setting, they either pass through a percentage of threat material, “missed detection” in security parlance, or they generate a high rate of false positives. CT scanner-based explosives detection systems are able to overcome problems of superimposition effects that arise in line scan systems. CT measures average x-ray absorption per voxel in slices projected through suspect regions of a bag. This parameter is not sufficiently specific to distinguish explosives from many other common materials. Items implicated in false positives include candy, various foodstuffs (e.g., cheese), plastics, and toys. Much attention has attended the deployment of CT-based explosives detection systems and their high false positive rate of around 30% in real world operating conditions is now well publicized in the media and has been acknowledged by the TSA. Concerns have been expressed about the resultant need to open and hand search a substantial portion of the checked bags, out of sight of the owner of the luggage. This is time consuming and expensive for the airlines and the prospect of airport delays and the potential for theft is a source of concern to the traveling public.
Moreover, CT scanners are unable to detect the presence of explosive material that is formed into thin sheets because CT averages the x-ray absorption coefficient over each voxel. Pentaerythritoltetranitrate (PETN), for example, will readily detonate when in the form of a sheet 1 mm thick. The density of PETN is 1.77 g/cc and a sheet 50 cm×50 cm×1 mm, easily incorporated into the skin of a suitcase, weighs approximately 442 grams, or almost 1 pound, which is sufficient to cause a powerful explosion.
Identification systems based on X-ray diffraction techniques provide enormously improved discrimination of materials. Such systems measure the d-spacings between the lattice planes of micro-crystals in materials. This form of energy-selective diffraction imaging has been employed in a type of medical tomography and in the non-destructive examination of pigments in works of art. X-ray diffraction provides a substance-specific fingerprint that greatly increases the probability of specific material detection and concomitantly reduces the incidence of false positives. Its applicability to explosives detection and the detection of other illicit substances has been demonstrated by Yxlon International of Germany with a prototype diffraction-based system.
Prior x-ray diffraction-based security systems for explosives detection and baggage scanning are not yet highly developed. These systems, such as Yxlon's system as illustrated in FIG. 1, are based upon work done by Bomsdorf and Muller at the University of Wuppertal in Germany. The Yxlon system utilizes small-area, single-crystal germanium (Ge) detectors. A divergent tight duster of collimated x-ray pencil beams, originating from an effective point source, is directed through the bag under examination and sensed by the high-purity Ge detector cooled to liquid nitrogen temperature of −196 degrees C. However, such a system suffers from a number of fundamental constraints. First, the high-purity Ge detector is too expensive to use in large area sensors. Second, the requirement for liquid nitrogen cooling is cumbersome and expensive to maintain in an airport environment. In addition, they can examine only a small area of a bag at one time due to the small detector size. This requires multiple passes of the beam through the bag being screened in which the beam is meander-scanned (zig-zag, back-and-forth pattern) through the bag in order to inspect the entire contents of the bag. This is too slow for volume applications like routine baggage scanning. Tests done by the Canadian Customs on detecting concealed samples of heroin and cocaine, which for a diffraction system is an equivalent task to identifying an explosive compound, have indicated scan times of the order of 1.5 minutes, far longer than the desired 6 seconds per bag.